A preferred embodiment of the invention is directed to a catalytic gasification method for removing or reducing coke deposits in cyclones of fluid cokers and/or on accompanying surfaces such as stripper sheds.
Fluidized bed coking (fluid coking) is a petroleum refining process in which mixtures of heavy petroleum fractions, typically the non-distillable residue (resid) from fractionation, are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 900to 1100xc2x0 F. (about 480to 590xc2x0 C.). A large vessel of coke particles maintained at the reaction temperature is fluidized with steam. The feed is heated to a pumpable temperature, mixed with atomizing steam, and fed through a plurality of feed nozzles to the fluidized bed reactor. The light hydrocarbon products of the coking reaction are vaporized, mixed with the fluidizing steam and pass upwardly through the fluidized bed into a dilute phase zone above the dense fluidized bed of coke particles. The transition between the dense bed (dense phase zone) and dilute phase, where product vapor is substantially separated from solid particles, is hereinafter referred to as the phase transition zone. The remainder of the feed liquid coats the coke particles and subsequently decomposes into layers of solid coke and lighter products which evolve as gas or vaporized liquid. The solid coke consists mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements. The fluidized coke is circulated through a burner, where part of the coke is burned with air to raise its temperature from about 900xc2x0 F. to about 1300xc2x0 F. (about 480to 704xc2x0 C.), and back to the fluidized bed reaction zone.
The mixture of vaporized hydrocarbon products and steam continues to flow upwardly through the dilute phase at superficial velocities of about 3to 6feet per second (about 1 to 2 meters per second), entraining some fine solid particles. Most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclone separators, and are returned to the dense fluidized bed by gravity. The gas phase undergoes pressure drop and cooling as it passes through the cyclone separators, primarily at the inlet and outlet passages where the velocity is increased. The cooling which accompanies the pressure decrease causes condensation of some liquid which deposits on surfaces of the cyclone inlet and outlet. Because the temperature of the liquid so condensed and deposited is higher than about 900xc2x0 F. (about 480xc2x0 C.), coking reactions occur there, leaving solid deposits of coke. Coke deposits also form on the reactor stripper sheds, and other surfaces of the fluid coker reactor.
The mixture of steam and hydrocarbon vapor is subsequently discharged from the cyclone outlet and quenched to about 750xc2x0 F. (about 400xc2x0 C.) by contact with downflowing liquid in a scrubber vessel section of the fluid coker equipped with internal sheds to facilitate contacting. A pumparound loop circulates condensed liquid to an external cooling means and back to the top row of scrubber sheds to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the fluidized bed reaction zone, but may be present for several hours in the pool at the bottom of the scrubber vessel and the pumparound loop, allowing time for coke to form and deposit on shed surfaces because of the elevated temperatures.
Feed is injected through nozzles with atomizing steam into the fluidized bed reactor. The feed components not immediately vaporized coat the coke particles and are subsequently decomposed into layers of solid coke and lighter products which evolve as gas or vaporized liquids. During this conversion process some coke particles may become unevenly or too heavily coated with feed and during collision with other coke particles stick together. These agglomerated, now heavier, coke particles may not be efficiently fluidized by the steam injected into the bottom of stripper section and are subsequently carried under from the reactor section to the stripper section where they adhere to and build up on the top rows of sheds in the stripper section. Build up of deposits on the stripper sheds can become so severe due to overlapping of the deposits on adjacent sheds as to restrict fluidization of the coke in the reactor section above and eventually shut the unit down.
Fouling of cyclone outlets and of stripper sheds in a Fluid Coker results in decreased throughput and eventual shutdown of the unit. Both effects can be very costly to a refinery. The deposits are sometimes removed from the outlet of the cyclone with metal rods and water jets at high pressure to mechanically clear the cyclone outlet area and to keep the unit running. The effectiveness of this approach is temporary and unpredictable. Chunks of coke may fall back into the cyclone body and interfere with cyclone operation. The coke deposits must also be removed from the reactor stripper sheds and other areas of the fluid coker that become fouled.
What is needed in the art is an efficient, predictable, and effective way to remove or reduce such detrimental coke deposits in fluid coker cyclones and accompanying surfaces to avoid throughput reductions and expensive shutdowns.
A preferred embodiment of the invention is directed to catalytic removal or reduction of coke deposits formed in a fluid coker unit during operation of said unit. Though the method is particularly useful for fluid coker units, it can be broadly applied to any units in which coke deposition occurs such as Fluid Catalytic Cracking Units (FCCUs). All that is necessary is that the coke deposits be accessible to reactant gas and that the metallurgy of the system be compatible with the catalytic gasification temperatures.
An embodiment of the invention is directed to a method for removing or reducing coke deposits in a refinery reactor unit, said method comprising catalytically gasifying said coke deposits by (a) optionally ceasing hydrocarbon feed to said unit, (b) coating or impregnating said coke deposits with a catalyst effective in converting coke to a gaseous product comprising hydrogen and carbon monoxide, (c) contacting said coke deposits with a reactant gas comprising substantially steam, in the substantial absence of oxygen, at a temperature of at least about 500xc2x0 C. for a time sufficient to convert a portion of said coke deposits to a gaseous product comprising substantially carbon monoxide and hydrogen.
In a fluidized refinery unit, fluidization of the coke may be maintained during said catalytic gasification. It may be necessary to temporarily reduce the flow of feed and/or fluidizing steam. One skilled in the art can readily determine how much and if the flow should be reduced based on the unit""s operating conditions. In the instant process, reactant gas comprising substantially steam may be added in addition to the steam utilized to fluidize the unit. The fluidizing steam may be incapable of reducing or removing coke deposits unless it is at a sufficiently high temperature. If the temperature of the fluidizing steam can be increased to the temperatures (at least about 500xc2x0 C.) described herein, then the fluidizing steam may be used as the gasifying steam.
As used herein comprising substantially steam means at least 99 volume % steam. In the substantial absence of oxygen means less than 1 volume % oxygen. Comprising substantially carbon monoxide and hydrogen means the gaseous product excluding steam, carbon dioxide and oxygen from combustion, and light hydrocarbon products-cracked off the coke will contain at least 90 volume % of carbon monoxide and hydrogen combined.